1. Field of the Invention
This invention relates to a coating apparatus for coating a surface of a work, and more particularly to a rotational coating apparatus in which a coating composition is applied to the surface of a work to a thickness larger than a limit thickness of running or sagging and the work is rotated about a substantially horizontal axis in order to prevent running or sagging of the coating composition.
2. Related Art
It has been known that when a work such as a vehicle body is coated with a coating composition, the smoothness of the coating film surface can be improved by increasing the amount of coating composition to increase the thickness of the coating film.
That is, when a coating composition is applied to the surface of a work, the surface of the coating film tends to become smooth due to its surface tension which acts on the surface of coating film as a tensile force in a direction parallel to the surface of the coating film. The smoothening effect by the surface tension becomes better as the flowability of the coating composition increases. The flowability of the coating composition increases with increase in the amount of coating composition (the thickness of the coating film). Accordingly, when the amount of the coating composition is increased, the flowability of the coating composition increases and the smoothness of the coating film becomes better. Especially when the coating composition is applied to a thickness larger than a limit thickness of running or sagging, the surface of the coating film becomes excellently smooth. The "limit thickness of running or sagging" means a minimum thickness of the coating film over which running or sagging of the coating composition will occur and will be referred to simply as "the running limit thickness", hereinbelow.
When the coating composition is applied to a work having a surface extending substantially in a vertical direction to a thickness larger than the running limit thickness, sagging or running of the coating composition occurs in the vertical surface of the work under gravity, which greatly deteriorates the smoothness of the coating film.
However when the work is rotated about a horizontal axis after application of the coating composition, a force in a direction opposite to the gravity acts on the coating composition on the vertical surface and sagging or running of the coating composition can be prevented. Further a tensile force which acts on the coating composition in a direction parallel to the surface of the coating film is generated by the rotation of the work, and the tensile force is associated with the surface tension of the coating composition to further smoothen the surface of the coating film.
Recently, as disclosed for instance in U.S. Pat. No. 4,874,639, there has been proposed a method of coating which is called "rotational coating method" and in which a coating composition is applied to the surface of a work to a thickness larger than a running limit thickness and the work applied with the coating composition is rotated about a substantially horizontal axis to prevent running or sagging of the coating composition from a time before the coating composition begins to run or sag until the coating composition sets to such an extent that the coating composition cannot run or sag, thereby obtaining a sufficient thickness of coating film to improve smoothness of the coating film surface while preventing running or sagging of the coating composition.
In the rotational coating apparatus, a thermosetting coating composition containing therein a solvent is generally used, and the rotational coating apparatus generally comprises a coating station including a coating zone for applying the coating composition to a work to a thickness larger than a limit thickness over which the coating composition on a surface of the work extending in a vertical direction will normally run or sag, a setting station including a setting zone for evaporating the solvent in the coating composition applied to the work (the setting zone is sometimes omitted) and a heat-hardening station including a heat-hardening zone for hardening the coating composition by heating after the setting in the setting zone, a rotating means for rotating the work about a substantially horizontal axis after the coating in the coating zone to prevent the coating composition on the vertical surface of the work from running or sagging until the coating composition comes into such a state that running or sagging cannot occur, and a conveyor means for passing the work through the coating zone, the setting zone and the heat-hardening zone in this order.
The coating composition is applied to the work in a plurality of times and is generally applied to a thickness larger than the running limit thickness in the final coating zone. For example, when the coating zone comprises first and second coating zones respectively for effecting first coating and second coating, the coating composition is applied to a thickness smaller than the running limit thickness in the first coating zone and then is applied to a thickness larger than the running limit thickness in the second coating zone.
In such a case, when the conveyor means comprises a single conveyor, the following problem is involved. That is, when some trouble, such as failure in a coating robot or the conveyor, occurs in a part of the coating line upstream of the zone where the coating composition is applied to a thickness larger than the running limit thickness (e.g., the second coating zone) and the coating line is stopped, said single conveyor is stopped in whole and accordingly the work which has been applied with the coating composition to a thickness larger than the running limit thickness is stopped there, which results in run or sag of the coating composition and defect in coating.
In order to avoid the problem, it has been proposed to separate the conveyor line between the first coating zone and the second coating zone and to drive the first conveyor on the side of the first coating zone and the second conveyor on the side of the second coating zone separately from each other as disclosed, for instance, in Japanese Unexamined Patent Publication No. 4(1992)-114757. This arrangements permits the second conveyor to continuously convey the work which has been applied with the coating composition in a thickness larger than the running limit thickness into the zones where the work is rotated and running or sagging of the coating composition is prevented (e.g., the setting zone and/or the heat-hardening zone) even if some trouble occurs in the first coating zone or the coating line upstream thereof.
Since the rotational coating apparatus is for obtaining a coating of high quality having a extremely smooth coating film surface, it is required a high provision against reduction in smoothness of the coating film surface due to running or sagging generated by trouble in the coating line or the like and reduction in the coating quality due to adhesion of dust or the like.
From this viewpoint, the conventional rotational coating apparatus where the conveyor line is separated between the first coating zone and the second coating zone still has the following problem.
That is, some trouble can occur in the downstream side of the second coating zone. In this case, though it is needless to say that the work cannot be conveyed downstream of the part of the trouble, it can be possible to convey the work which has been applied to a thickness larger than the running limit thickness out the second coating zone into the rotating zones where the work is rotated and running or sagging of the coating composition is prevented (e.g., the setting zone and/or the heat-hardening zone) or a position where the coating composition has come into such a state that running or sagging cannot occur, so long as the part of the trouble is downstream of the rotating zones and the position where the coating composition has come into such a state that running or sagging cannot occur and the conveyor itself can convey the work downstream from the second coating zone.
However when the conveyor downstream of the second coating zone is filled with the works, the work in the second coating zone cannot be conveyed out the second coating zone and must stay there, which results in running of sagging of the coating composition and defect in coating.
Further in the rotary coating line, the work is generally conveyed on a rotary truck and is rotated on the rotary truck in the zones where running or sagging can occur. When dust and the like are on the rotary truck, the dust and the like fly and adhere to the coating film surface when the work is rotated, which results in a low quality of coating. Further when the mechanism for rotating the work gets trouble and the work cannot be rotated in a proper manner, the smoothness of the coating film surface deteriorates.